Preheat the oven to 325 F (165 C). In a medium bowl combine the flour, cocoa, baking soda and salt. Set aside.
Melt the butter and 4 oz of semisweet chocolate together over low heat or bain-marie. Remove from heat and let cool slightly.
Combine the eggs and sugar and beat at medium speed with a hand mixer until light and fluffy, about 3-5 minutes. Add the chocolate mixture and stir until well combined. Add mint extract.
Add the flour mixture and stir until just combined. Fold in the chocolate chips or chunks.
Drop 1 tablespoon portions 2 inches (5cm) apart onto a baking sheet lined with parchment paper.
Sprinkle crushed candy canes on top of each.
Bake for 10-12 minutes — they should be slightly underbaked so the interior stays chewy and soft.
Let the cookies cool on the baking sheets until the cookies are firm enough to remove, probably another 5-10 minutes.
While the cookies are still warm, sprinkle extra crushed candy canes on top of each for a beautiful presentation. Allow them to cool completely on a wire rack.